1. Field of the Invention
This invention relates to a focus detector used in an optical equipment using an optical lens system such as an automatic focusing type single lens reflex camera (hereinafter abbreviated as AF-SLR camera).
2. Description of the Prior Art
A conventional focus detector for detecting a focus of an optical lens system is described with reference to FIGS. 50 and 51.
As can be seen from FIG. 50, the focus detector 20 is disposed behind an equivalent focal plane 11 of an optical lens system 10. The focus detector 20 comprises a condenser lens 21, an aperture mask 22, a pair of reforming lenses 23, a pair of photoelectric transfer device arrays 24 such as CCD (Charge Coupled Device) and an operation circuit 25 for calculating a quantity of defocus and for detecting a direction of movement of the optical lens system 10. The quantity of defocus is defined as a distance between an actual focus of the optical lens system 10 and the equivalent focal plane 11.
The quantity of defocus and the direction of the movement of the optical lens system are obtained from a relation of positions of two images focused on the photoelectric transfer device arrays 24. When the focus of the optical lens system 10 is coincides with an object, that is an image of the object is focused on the equivalent focal plane 11, a distance between the images respectively focused on the photoelectric transfer device arrays 24 becomes a constant value L0. When the image of the object is focused behind the equivalent focal plane 11, the distance between the images on the photoelectric transfer device arrays 24 is shorter than the constant value L0. Alternatively, when the image of the object is focused before the equivalent focal plane 11, the distance between the images on the photoelectric transfer device arrays 24 is longer than the constant distance L0. Thus, it is possible to know whether the image of the object is focused on the focal plane or not from the distance between the ima es focused on the photoelectric transfer device arrays 24. Furthermore, when the image of the object is not focused on the focal plane, the quantity of defocus which is a displacement for moving the optical lens system 10 and the direction of the movement of the optical lens system 10 can be obtained from the value of the distance.
In an AF-SLR camera system, many kinds of interchangeable lenses (which are finished goods including an optical lens system, an aperture mechanism and a lens moving mechanism) can be mounted on a camera body. In FIG. 51, solid lines designate optical paths of an optical lens system 10a having a relatively smaller open aperture F-number such as F2.3 or F4. Alternatively, dotted chain lines designate optical paths of an optical lens system 10b having a relatively larger open aperture F-number such as F5.6 or F6.7. As can be seen from FIG. 51, a diameter of a pupil of the optical lens system 10b having the larger open aperture F-number is smaller than that of the optical lens system 10a having the smaller open aperture F-number.
In order to automatically control the focus of the optical lens system of the AF-type interchangeable lens which can be mounted on the AF-SLR camera body, a pair of openings 22a of the aperture mask 22 are positioned near the optical axis corresponding to the diameter of the pupil of the optical lens system having the largest open aperture F-number (for example F6.7) in a group of interchangeable lenses. Thus, even when an AF-type interchangeable lens having the smaller open aperture F-number such as F2.8 or F4 is mounted on the camera body, focus detection is executed by using light flux which passes near the optical axis of the optical lens system. Light flux passing farther from the optical axis is wasted.
In a recent AF-SLR camera, a plurality of focusing areas are disposed in a frame for focusing objects positioned not only in the center portion of the frame but also in regions distant from the center of the frame. An arrangement of the focusing areas is shown in FIG. 52. In FIG. 52, a rectangular frame 25 designates a frame on the focal plane of a camera body or a frame of the focusing screen of a viewfinder. A first circular region 30 having a smaller diameter designates a region where light flux emitted from an object and passing through a pupil of an optical lens system having a larger F-number can reach. A second circular region 40 having a larger diameter designates a region where light flux emitted from an object and passing through a pupil of an optical lens system having a smaller F-number can reach.
Generally, when the position of the image focused on the photoelectric transfer device array 24 is distant from the optical axis, the movement of the image responding to the movement of the focus of the optical lens system 10 becomes larger. Thus, it is preferable that the light flux passing through the region positioned far from the optical axis is used to increase the accuracy of the focus detection. In addition, as the open aperture F-number of the optical lens system 10 becomes smaller, the the blur of the image due to the defocus becomes more significant. Thus, it is necessary to detect the focus of the optical lens system 10 accurately when the open aperture F-number of the optical lens system 10 is small.